Magento (as usual) has many options to do so. I will describe the simplest way.
Create the file view/<area>/layout/default.xml inside your module folder (app/code/Df/Core, for example).
Write «adminhtml» or «frontend» in place of <area>.
An example of default.xml:
<?xml version="1.0"?>
<page
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
layout="admin-1column"
xsi:noNamespaceSchemaLocation="../../../../../../../lib/internal/Magento/Framework/View/Layout/etc/page_configuration.xsd"
>
<head>
<link src="Df_Core::core.js"/>
<css src="Df_Core::core.css"/>
</head>
<body/>
</page>
Write your module name in place of Df_Core.
Place core.js и core.css to the folder view/<area>/web.
Bonus! You can add Less files too!
If you create core.less file instead of core.css, but refer it as <css src="Df_Core::core.css"/> in default.xml above, Magento 2 will intelligently and automatically compile your less file into css!
So you can use Less files in your modules!
See the official article CSS preprocessing for the more info.